Method for surface treatment of zinc-plated sheet steel

ABSTRACT

Method of the surface treatment of zinc-plated sheet steel, wherein cathodic electrolysis of the surface of zinc-plated sheet steel is provided as a pretreatment of the surface treatment with chromic acid of said zinc-plated sheet steel, for the prevention of white rust.

United States Patent Hideya Okada;

Shozo Matsuda, both of Kawasaki; Misao Ohbu, Kitakyushu, all of Japan [21] Appl. No. 837,331

[72] Inventors i [22] Filed June 27, 1969 [45] Patented Dec. 21, 1971 [73] Assignee Nippon Steel Corporation Tokyo, Japan [32] Priority June 30, 1967 [33] Japan [54] METHOD FOR SURFACE TREATMENT OF ZINC- PLATED SHEET STEEL 7 Claims, No Drawings [52] US. Cl 204/35 R, 204/40, 204/ 147 [51] 1nt.Cl C23f 13/00 [50] Field of Search 204/35 R,

Primary Examiner-John H. Mack Assistant Examiner-W. 1. Solomon Att0rney-Wenderoth, Lind & Ponack ABSTRACT: Method of the surface treatment of zinc-plated sheet steel, wherein cathodic electrolysis of the surface of zinc-plated sheet steel is provided as a pretreatment of the surface treatment with chromic acid of said zinc-plated sheet steel, for the prevention of white rust.

METHOD FOR SURFACE TREATMENT OF ZINC- PLA'IED SHEET STEEL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general to the method of the surface treatment of zinc-plated sheet steel, and more particularly to a novel method for the surface treatment of hot dipping zinc-plated sheet steel for the prevention of white rust.

2. Description of the Prior Art To gather white rust on the surface easily during storage and transportation, is the trouble with zinc-plated sheet steel; and in order to prevent the rust, there is generally carried out a simple rust prevention method according to which such steel is treated in a treating bath consisting mainly of chromic acid, so as to form chromate film on the surface. However, this conventional method is not effective enough in such humid areas as Japan; even the treated steel gathers white rust, that is, the prevention of the rust is difficult. The formation of white rust is the main reason of complaints about zinc-plated sheet steel.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for the surface treatment of zinc-plated sheet steel, as an improvement of the conventional method for the prevention of white rust formation on the surface of zinc-plated sheet steel, according to which improvement the treatment is simple and yet effective in the prevention of white rust formation during handling and storage, even in humid areas.

Another object of the present invention is to provide a pretreatment method to the conventional method for the surface treatment of zinc-plated sheet steel with chromic acid for the prevention of white rust formation, such pretreatment method making the surface treatment perfectly effective and yet being operated in a short time and easily.

In order to attain these objects, the method of the present invention is so practised that in treatinghot dipped zinc-plated sheet steel by immersion or electrolysis in a treating bath containing chromic acid alone or as the main ingredient for the prevention of white rust, cathodic electrolysis of the surface of said sheet steel is carried out in a diluted aqueous solution of such stable neutral salts as I (Cl, NaCl, (NH,),CO (NH,) Cr O, and (NH,,),CrO as the pretreatment for increasing the effect of the above treatment, thereby activating and reducing the surface of said steel by having the zinc oxide film covering the surface reduced to be removed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Having studied minutely the reaction of chromic acid with the surface of zinc-plated sheet steel in the said treatment with chromic acid, the inventors discovered a 1,000 A. thick film of such zinc compounds as basic zinc carbonate covering the surface of hot dipped zinc-plated sheet steel on the market and hindering the above reaction with chromic acid.

Regarding such film, studies were carried out widely in the fields concerned, but there were presented no findings until the inventors of the present invention attained, through analysis of electronic diffractions, a finding that such harmfulfilm consists mainly of basic zinc carbonate and noncrystalline zinc oxide. Based on this finding, the inventors tried a variety of methods for the removal of oxides from the surface of hot dipped zinc-plated sheet steel, and have found that if cathodic electrolysis is provided to the treatment with chromic acid so as to remove the above-mentioned oxides with scarce change to the appearance of the surface, even the conventional chromate treatment using the simple process of immersion can produce, uniformly over the surface and efficiently, a chromate film better in white rust prevention than that made by only the treatment with chromic acid but without this pretreatment according to the conventional method.

The present invention is characterized by subjecting the surface of hot dipped zinc-plated sheet steel to cathodic electrolysis in an aqueous solution of at least one kind selected from a (NHOZCO; and (N H4)2Cr O as mentioned above, with comparatively diluted concentration with a pH value of about 6 to l0, using the hot dipped zinc-plated sheet steel as cathode, at a current density of less than 50 A./dm. more preferably l to 25 A./dm. for such a short time of I second; it is to activate and reduce the surface with scarce change to the appearance, thereby efficiently reducing and removing zinc oxide and other materials coveringthe surface of steel as mentioned above.

When using pretreatment according to the present invention, the reaction of chromic acid with the surface of the steel is greatly promoted, making even the conventional chromate treatment, using the simple immersion done subsequently, useful in the production of a chromate film effective for the prevention of white rust formation in humid areas, as mentioned below in examples; even better results could be obtained in the prevention of white rust with a chromate film produced by the more elaborate process of electrolysis used for the conventional chromate treatment.

The reason that, according to the present invention, cathodic electrolysis is used for the removal of oxide film from the surface of zinc-plated sheet steel, as mentioned above, and that the electrolyte used for such electrolysis is limited in pH value to a range from 6 to 10, is explained in comparison with other methods, as follows:

The above removal of oxide film from the surface of zincplated sheet steel makes it easy to carry out the chromate treatment done posterior to the removal, and the treated steel is greatly improved in the corrosion resistance and the white rust prevention of the surface zinc.

When considering the removal of oxide film from the surface zinc from a general viewpoint, the simplest and easiest process is to dissolve away such .oxide film chemically, and that can be done effectively by using a highly acidic solution of low pH value, or, on the contrary, a highly alkaline solution of high pH value, when the following reactions take place:

pH l2: ZnO-l-OH-'HZnO In these cases, zinc oxide film is dissolved away in the state of ion.

However, with such pH values as mentioned above, metallic zinc also dissolves easily; therefore, with the above pH values, that is, when using a solution of HCl or NaOH, the oxide film on the surface zinc dissolves away, but troubles are that the soexposed metallic zinc also dissolves, thus the zinc plating layer is damaged and zinc ions accumulate-in the solution. Because of such troubles, the use of acid or alkali for the removal of oxide film is not recommendable. Another trouble is that if such treatment is carried out continuously, a part of acidic or alkaline solution is mixed with a chromic acid treating solution, causing undesirable results.

When using cathodic electrolysis according to the present invention, the oxide film can be removed effectively without such troubles. By reduction by such electrolysis, oxide and basic zinc carbonate film can be removed. As for the so-exposed surface zinc, it does not dissolve, as it is held with anode and the solution is limited in pH value to a range of 6 to 10. The noteworthy merit of this process is that the oxide film covering the surface is removed without damaging the zinc plating layer.

As mentioned above, the electrolyte should not be strongly corrosive, as the surface metal has been exposed as a result of the reduction made by the electrolysis for the removal of the oxide film. Taking this point into consideration, pH value of the electrolyte is desirable to be in a range of 6 to 10, so that the metallic zinc is dissolved with difficulty. For making such electrolyte, a solution made of such a substance as KCl, NaCl,

(NHJ CrO, and (NH.),Cr,0, which makes a pH value of6 to Table I shows the comparison in corrosion resistance after 10, in water is used, Again regarding the limitation ofpH value the treatments, using an aqueous solution of chromic acid to maximum of 10, it is because zinc in a solution of a pH alone and a bath containing chromic acid as the main invalue of more than 10 is apt to turn into the passive state; gredient, between the samples previously removed of oxide therefore, oxide film forms again during the period between 5 film according to the present invention and those of the same reduction by electrolysis and rinsing the water; hence the pH specifications but with oxide film left n v d,

value of an electrolyte effective for the removal of oxide film from the surface zinc by the reduction made by cathodic electrolysis is preferred to be between 6 to 10. Moreover, elec- AMPLE 2 trolysis in a solution of the pH value within this range, can be 10 carried out successfully at the current efficiency of 100 percent without generation of hydrogen.

Regarding the treatment with chromic acid which is to be Cathodic electrolysis according to the present invention was carried out under the following conditions:

used in combination with the pretreatment according to the 51mm: f f r 'l i present invention, no explanation of it is given, as it is publicly l5 2. Electrolyte: 0.1 N ammonium bichmmt, known. However, it should be noted that both immersion and solution.

electrolysis can be applied to the above treatment; also use of 3:21:12? fir :3;

any ofa treating bath containing chromic acid or as a main in- Tempcmm. WC

gredient or a bath containing chromic acid plus such other substances as silica gel for the above treatment can be ex- EXAMPLE 3 pected to bring about satisfactory results as mentioned below. The following examples l-4 show the comparison in corrosion resistance after the same treatment with chromic acid between the samples of hot dipped zinc-plated sheet steel with El I I s P oxide film left unremoved and those of the same specifications I Electrodes Same as mentioned solution but removed of oxide film by the pretreatment according to 3. Current density: 10 AJdm. th present i ti 4. Operation time: I second 5. Temperature: 40' C.

EXAMPLE 1 EXAMPLE 4 removaiof oxide mm was F f out f) caihodic F When treated with an aqueous solution of chromic comtrolysis according to the present invention, with the following pounds (an clectrolyte requiring no water rinsing) conditions:

l. Electrodes: Anode: Stainless steel lv Electrodes: Cathode: Hot dipped zincplate plated sheet steel Cathode: Hot dipped zinc- Anode: Carbon plate plated sheet steel 2. Electrolyte: 0.l N ammonium carbonate 2. Electrolyte: 0.l N ammonium bicl'tromate solution 0.1 N chromate anhydride 3. Current density: 5 AJdm. 40 3. Current density: 10 A/dm 4. Operation time: l-2 seconds 4. Operation time: I second 5. Temperature: Room temperature 5. Temperature: SOC.

TABLE 1.-COMPARISON IN CORROSION RESISTANCE White rust (area percent) produced by brine Chromium spray tests afterdeposited Sample No. Pretreatment Chromie acid treatment (mm/in?) 24 hrs. 48 hrs. 72 hrs. 96 hrs.

1 (Con.)..-. No ..}Immersed in aqueous solution of chromic 100 100... 100 1 E (Pl). Reduction by electroly is. acid alone. No change... 5... 20 2 (Con.).. No ..}Immerst d in bath containing cltromic ucid B 1 100 2 E (PI).... Reduction by electrolysis. as main ingredient. No change" No 6 Remarks: (1) Bath containing chromic acid (2 and 2E): Chromic auhydride l5 g./l., colloidal silica 20 g./l., wuter rest, solution temperature 25 C.

(2) Con.=By conventional method; 1 1=Accordiuu to the present invention.

TABLE 2.COMPARISON 1N CORROSION RESISTANCE Whito rust (area percent) produced by brine Chromium spray tests after-- deposited Sample No. Pretreatment Chromic acid treatment (mgJmfl) 24 hrs. 48 hrs. 72 hrs. hrs.

1 (Con.) No .}Imrnersed in aqueous solution of chromic 8 b0. 100 100 1 E (PI). Reduction by electrolysis. acid alone. 10 No change... No change. 5 10 2 (Con.) No... ..}Immcrsed in bath containing chromic acid I 16 6. 100 100 100 2 E (PI)... Redu by electrolysis. as main lugredieut. 1 14 No change. No change... No change" 5 Remarks: (1) Bath containing chromic acid (2 and 2E): Same as mentioned in Example 1. (2) Cou.= lly conventional method; ll=At'cortiing to the present invention.

TABLE 3 White rust (area percent) produccd by brino sprny tests after- Total Cr Sample No. Pretreatment Chromic acid treatment deposited 24 hrs. 48 hrs.

1 (Com). No... }il1IlIt(lSl(lil1 aqueous solution of chromic l1('i(i its main ingredientsqueeze {til IIltL/lll- No change '20. l E (li).. lllvctrolysis... roll. ii) NHL/111.2 .(io. No ('illlllgl.

itvnuirks: (i) llnth containing r-hrotnlt: ltt'iti (L! znnl 2E): h'nnuus nn-ntltmutl in ltxntnple l. (2) ou/ 15) t'ollvtlltlullnl method; ll=At-t'ortllng lo the present. invention.

TABLE 4 White rust (urea percent) pro (lured by brine spray tests after Total Cr a Sample No. Pretreatment Electrolyte for pretreatment Chromic acid treatment deposited 24 hrs. 48 hrs. 1(Con.) \1% ehromate anhydride aqueous solu- (8 rug/m. 60. 100, 1 E (PI) Electrolysis 0.1 N chromate anhydride ...1 tion (squeeze roll). [8 rug/n1. No change 10. 2 (Con. i No u }Aqueous solution containing eln'omate {IUlllL/m- 4lo 20. 2E (PI) Electrolysis." 0.1 N ammonium blchromate asmain ingredient (squeeze roll). ln1g./m. do. No change.

Remarks: (1) Bath containing chromic acid (2 and 2E): Same as mentioned in Example 1. (2) Con.=lly conventional method; ll=Act-ording to the present invention. (3) ately that is, not through water rinsing.

chromic acid in forming chromate film on the surface for the prevention of white rust formation wherein the said treatment is preceded by a pretreating process, the improvement according to which said pretreating process comprises a preliminary treatment by cathode electrolysis of the surface of the zincplated steel sheet in an electrolytic solution made of a diluted.

water solution of a stable neutral salt.

2. The method according to claim 1 wherein said cathodic electrolysis is carried out in a diluted aqueous solution of at least one stable neutral salt selected from the group consisting of KCl, NaCl (NH CO (Nl-LhCrO, and (NH.),Cr,O

When using this pre-treatment according to the present invention, the treatment with chromic acid can he given immedi- 3. The method according to claim 1 wherein said cathodic electrolysis is carried out in an electrolyte of the pH value ranging from 6 to 10.

4. The method according to claim 1 wherein said cathodic electrolysis is carried out in the electrolyte of the pH value ranging from 6 to l0 and at a current density of 5 to 20 A./dm. for a period of l to 2 seconds.

5. The method according to claim 1 wherein the surface treatment with chromic acid is carried out in a aqueous solution either of chromic acid alone or containing it as the main ingredient.

6. The method according to claim 1 wherein the surface treatment with chromic acid is carried out by immersion.

7. The method according to claim I wherein the surface treatment with chromic acid is carried out by electrolysis. 

2. The method according to claim 1 wherein said cathodic electrolysis is carried out in a diluted aqueous solution of at least one stable neutral salt selected from the group consisting of KCl, NaCl (NH4)2CO3, (NH4)2CrO4 and (NH4)2Cr2O7.
 3. The method according to claim 1 wherein said cathodic electrolysis is carried out in an electrolyte of the pH value ranging from 6 to
 10. 4. The method according to claim 1 wherein said cathodic electrolysis is carried out in the electrolyte of the pH value ranging from 6 to 10 and at a current density of 5 to 20 A./dm.2 for a period of 1 to 2 seconds.
 5. The method according to claim 1 wherein the surface treatment with chromic acid is carried out in a aqueous solution either of chromic acid alone or containing it as the main ingredient.
 6. The method according to claim 1 wherein the surface treatment with chromic acid is carried out by immersion.
 7. The method according to claim 1 wherein the surface treatment with chromic acid is carried out by electrolysis. 